1. Field Of The Invention
The present invention relates to an oxygen sensor device. More particularly, the present invention is concerned with an oxygen sensor device having a glass dome fluid-tightly connected at its circumferential base edge to one surface of an oxygen conductive solid electrolyte plate to form a diffusion chamber defined by the glass dome and the solid electrolyte plate. The oxygen sensor device is advantageous from the viewpoints of miniaturization, power consumption saving and reliability. The advantageous oxygen sensor device can be efficiently produced by a novel method, in which a burnable material layer, a porous antifire material layer or a combination of a burnable material layer and a porous antifire material layer is formed on an oxygen conductive solid electrolyte plate and a powdery glass-containing pasty material is coated thereon, followed by firing. By virtue of the novel method, it is possible to perform continuous mass production of the oxygen sensor device at low cost, with the minimized occurrence of defectives.
2. Discussion Of Related Art
Oxygen determining devices are required when human beings must work in confined spaces, such as mines, tanks, etc. Oxygen determining devices are also requisite in extremely diversified fields, such as space ships and capsules, submarines for naval and civilian use, medical treatment, food pack inspection as well as combustion control and other environmental studies. Although several types of oxygen sensor devices have been proposed, there is still a strong demand for a small reliable oxygen sensor device.
A representative form of the conventional oxygen sensor device is shown in FIG. 1. The device comprises diffusion housing 104, oxygen conductive plate 101 of solid electrolyte hermetically bonded at its one surface to diffusion housing 104 by sealing glass 107, and a pair of circular electrode layers 102A and 102B secured to the opposite sides of conductive plate 101, respectively. Electrodes 102A and 102B are connected to a DC power source. For example, the oxygen conductive plate 101
comprises a solid solution containing ZrO.sub.2, Y.sub.2 O.sub.3, MgO and CaO. Each of electrode layers 102A and 102B is porous and is made of, for example, platinum. Diffusion housing 104 has orifice 105 (gas inlet means) of a small diameter formed through the wall of the housing. The diffusion of oxygen from the monitored gas environment through gas inlet means 105 into chamber 103 of diffusion housing 104 is effected by the application of a DC potential from a power source across two electrodes 102A and 102B to pump the oxygen present in chamber 103 through oxygen conductive plate 101. As the potential across two electrodes 102A and 102B is increased, electrical current flowing through two electrodes 102A and 102B is changed. The current limited by the oxygen diffusion becomes stable so that a stable diffusion limited current value is obtained. The diffusion limited current value is proportional to the concentration of oxygen in the monitored gas environment, and therefore the oxygen concentration can be detected by measuring the diffusion limited current value through a current meter (see, for example, U.S. Pat. No. 4,571,285).
In another conventional oxygen sensor device, the diffusion housing is made of an open-cell porous structure, with the gas inlet means omitted. With this construction, the oxygen diffuses into the chamber defined by the housing and the electrolyte plate through the pores of the housing.
Generally, it is desired that the oxygen sensor device have a small size, because, with the small size, it is easy to heat up the device and perform temperature control at the time of oxygen determination, and the oxygen sensor device can be applied to oxygen determination even when the space at the oxygen determination site is limited. In manufacturing the above-mentioned conventional oxygen sensor devices, miniaturization of the device to a size of some millimeters is accompanied by difficulties in the fabrication and assembly of components, such as housing and solid electrolyte. For example, diffusion housing 104 is made of a hard material, such as that prepared by firing a ceramic. It is very difficult to rework such a hard material into a diffusion housing of small size which has complicated configuration. Further, it is also very difficult to fluid-tightly connect such a small diffusion housing to a solid electrolyte plate. In particular, the connection of the diffusion housing to the solid electrolyte plate is carried out by applying sealing glass and then melting the glass in a high-temperature oven. However, this operation is likely to cause positional displacement, thereby leading to a high ratio of defectives. Because of the above-mentioned difficulties, the conventional oxygen sensor devices is likely to lack reliability in quality, and the productivity of the devices is disadvantageously low.
U.S. Pat. No. 4,762,605 discloses an improved method for manufacturing an oxygen sensor device In the improved method, a green sheet is used. The terminology "green sheet" used herein means a flexible plate or sheet comprising ceramic particles bound with a plasticizer, such as that prepared by first blending together a powdery ceramic, an organic resin plasticizer, e.g., polyethylene glycol, and a solvent, e.g., water and an alcohol, secondly extruding the resultant blend through rolls or a slit into a sheet and thirdly evaporating the solvent at a temperature, e.g. 100.degree. to 500.degree. C. A representative mode of this improved method is described below. First, a layer of a burnable resin material is disposed on the cathode side of an oxygen conductive solid electrolyte plate sandwiched between a porous cathode and a porous anode. Secondly, a pin-shaped protrusion made of the same resin material or of a hard plastic or an aluminum filament each having a low melting temperature is embedded in the layer of the burnable resin material. Thirdly, the above-mentioned green sheet is disposed on the layer of the burnable resin material. At this time, the protrusion passes through the layer. Finally, firing is performed. As a result, the plasticizer, the resin material and 5 the hard plastic are burnt, and the aluminum filament is melted off. Thus, an oxygen sensor device comprising an oxygen conductive solid electrolyte plate sandwiched between a porous cathode and a porous anode and a ceramic diffusion housing connected at its circumferential base edge to the cathode side of the solid electrolyte plate, is obtained, having a similar structure to that of FIG. 1. In this method, the firing must be performed at a temperature as high as about 1600.degree. C. This high temperature firing is likely to damage the pore structure of the cathode and anode, because the pore structure of each electrode is adversely affected at a temperature of higher than 1000.degree. C. Also, the temperature as high as about 1600.degree. C. is likely to cause deformation of the electrolyte plate. Therefore, it is very difficult to produce continuously such a type of oxygen sensor with uniform, excellent quality on a commercial scale.